1. Field of the Invention
The present invention relates to an image forming apparatus including a light scanning apparatus.
2. Description of the Related Art
Hitherto, an electrophotographic image forming apparatus such as a digital copying machine, a laser beam printer, or a facsimile apparatus includes a light scanning apparatus. The light scanning apparatus scans a surface of a photosensitive member with a light beam to form an electrostatic latent image on the photosensitive member. The light scanning apparatus includes a light-receiving element configured to receive the light beam deflected by a rotary polygon mirror. The light-receiving element generates a synchronous signal by receiving the light beam. The image forming apparatus emits the light beam based on image data with reference to the synchronous signal, to thereby match scan starting positions of the light beam in a scanning direction thereof with one another among respective scans. There is proposed a light scanning apparatus including, in order to determine an emitting timing of the light beam to be entered into the above-mentioned light-receiving element, a detection unit configured to detect a rotation phase of the rotary polygon mirror and output a rotation phase signal.
The detection unit includes a mark formed on a rotor flange to which the rotary polygon mirror is fixed by a pressing force of a spring, and a sensor configured to irradiate the mark with light and detect the reflected light. The sensor is configured to generate a detection signal indicating that the mark has been detected when the rotor flange is rotated to cause the mark to reach a detection area of the sensor. During pre-rotation processing at a time of warm-up, the CPU uses an internal counter of the CPU to measure a time period tp from a detection signal of the sensor up to the synchronous signal of the light-receiving element, and stores the time period tp in a memory. Note that, even when the light beam is emitted in order to obtain the synchronous signal from a beam detector during the pre-rotation processing at the time of the warm-up, marking back does not occur on a recording medium because an image is not being formed. At normal image formation, in a state in which a speed of the rotary polygon mirror is constant, after a time period (tp−α) has elapsed from an initial rotational position signal, the light beam is emitted from a light source to generate the synchronous signal by the beam detector. Here, α is an allowance estimated as a time period in which the light beam does not reach the photosensitive member while synchronization is positively enabled in consideration of an effective scanning periodic ratio. In this manner, the light beam for generating the synchronous signal is emitted at a timing that does not affect exposure of the photosensitive member, and hence a light emitting timing of the light beam for generating the synchronous signal can be determined without causing the marking back to occur on the recording medium (Japanese Patent No. 3,694,164).
However, according to Japanese Patent No. 3,694,164, in order to detect a rotational position of the rotary polygon mirror, the mark formed on the rotor flange and a light reflection sensor configured to detect the reflected light from the mark need to be separately provided within the light scanning apparatus. This may increase the number of parts that form the light scanning apparatus, and causes an increase in cost.